OMAPDSS: DISPC: Pipe threshold sizes power optimization.

This patch changes the calculation of thresholds and sets low
threshold to half of buffer size for OMAP4. The memory will
able to sleep between transactions to dss because dss will
read by big chunks.

boype: Also removed dispc_calculate_threshold function, as it is no longer used.

Backport from TI omapzoom kernel 3.4 repo, based upon:
http://git.omapzoom.org/?p=kernel/omap.git;a=commit;h=cd99eb345e1ad26ffbc665f46ea917dad762937b

Signed-off-by: Dan Trifan <jimsth@gmail.com>